**Core Concept**
Birbeck granules are a type of organelle found in the cytoplasm of Langerhans cells, which are a subset of dendritic cells. These granules are characterized by their distinctive tennis racket or rod-like appearance under electron microscopy.

**Why the Correct Answer is Right**
Birbeck granules are involved in the processing and presentation of antigens to T-lymphocytes. They contain a protein called Langerin, which is essential for the formation and function of Birbeck granules. Langerin is a C-type lectin that plays a crucial role in the capture and processing of pathogens, leading to the presentation of antigens to T-cells. The presence of Birbeck granules is a diagnostic feature of Langerhans cell histiocytosis, a group of disorders characterized by the proliferation of Langerhans cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Birbeck granules are not a feature of plasma cells, which are involved in the production of antibodies.
* **Option B:** This option is incorrect because Birbeck granules are not a feature of macrophages, which are involved in the phagocytosis of foreign particles and dead cells.
* **Option C:** This option is incorrect because Birbeck granules are not a feature of mast cells, which are involved in the immune response and are characterized by the presence of histamine.

**Clinical Pearl / High-Yield Fact**
Birbeck granules are a diagnostic feature of Langerhans cell histiocytosis, which can present with a wide range of symptoms, including skin rashes, bone lesions, and respiratory problems. A high index of suspicion is required to diagnose this condition, particularly in patients with unexplained symptoms or laboratory abnormalities.

**Correct Answer: C. Langerhans cells.**